Arrangement of photonic chip and optical adaptor for coupling optical signals

ABSTRACT

An apparatus includes an optical adaptor having monolithically integrated optical elements and first micro-mechanical features, the latter defining at least a first horizontal reference surface and a first vertical reference surface; wherein the first horizontal reference surface is perpendicular to an optical plane, the latter being perpendicular the optical axis of the optical elements; and wherein the first vertical reference surface is perpendicular to the first horizontal reference surface and parallel to the optical axis.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.14/017,610, filed Sep. 4, 2013, which claims priority to Great BritainPatent Application No. 1217383.7, filed Sep. 28, 2012, and all thebenefits accruing therefrom under 35 U.S.C. §119, the contents of whichin its entirety are herein incorporated by reference.

BACKGROUND

The present invention relates to the field of aligning opticalcomponents, notably optical coupling elements, to optical waveguides.

A high optical coupling efficiency between a photonic chip—or photonicintegrated circuit (PIC)—and an optical element is a key requirement inthe optical industry. Positioning errors may lead to sub-optimalcoupling and therefore to optical loss which impair the overall systemperformances.

U.S. Pat. No. 7,415,184 discloses an arrangement for providing opticalcoupling into and out of a relatively thin silicon waveguide, formed inthe SOI (silicon-on-insulator) layer of a SOI layer stack and includinga lens element and one defined reference surface within the SOI layerstack for providing optical coupling. However, no passive lateralcoupling can be realized because no vertical reference surfaces areavailable. Thus, a need remains in art for an efficient arrangement forcoupling optical signals between waveguides and another optical element.

SUMMARY

In one embodiment, an apparatus includes an optical adaptor havingmonolithically integrated optical elements and first micro-mechanicalfeatures, the latter defining at least a first horizontal referencesurface and a first vertical reference surface; wherein the firsthorizontal reference surface is perpendicular to an optical plane, thelatter being perpendicular the optical axis of the optical elements; andwherein the first vertical reference surface is perpendicular to thefirst horizontal reference surface and parallel to the optical axis.

In another embodiment, a system for coupling optical signals includes anoptical adaptor having monolithically integrated optical elements andfirst micro-mechanical features, the latter defining at least a firsthorizontal reference surface and a first vertical reference surface,wherein the first horizontal reference surface is perpendicular to anoptical plane, the latter being perpendicular the optical axis of theoptical elements, and the first vertical reference surface isperpendicular to the first horizontal reference surface and parallel tothe optical axis; and a photonic chip, comprising waveguides havingwaveguide terminations at a trench of the photonic chip, and secondmicro-mechanical features defining at least a second horizontalreference surface and a second vertical reference surface; wherein thesecond horizontal reference surface is perpendicular to a second opticalplane, the latter being perpendicular to a second optical axis of thewaveguides, the second vertical reference surface is perpendicular tothe second horizontal reference surface and parallel to the secondoptical axis, the first horizontal reference surface is in contact withthe second horizontal reference surface and the first vertical referencesurface is in contact with the second vertical reference surface.

In another embodiment, a method is disclosed for aligning opticalelements of an optical adaptor according to with waveguides of aphotonic chip, the optical adaptor monolithically integrating opticalelements and first micro-mechanical features, the latter defining atleast a first horizontal reference surface and a first verticalreference surface, wherein the first horizontal reference surface isperpendicular to an optical plane, the latter being perpendicular theoptical axis of the optical elements, and the first vertical referencesurface is perpendicular to the first horizontal reference surface andparallel to the optical axis. The method includes putting in contact thefirst horizontal reference surface and the first vertical referencesurface with corresponding second horizontal surface and second verticalsurface of the photonic chip.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The above and other aspects of the features of the present inventionwill become readily apparent from the detailed description that follows,with reference to accompanying drawings, in which:

FIG. 1 illustrates an optical adaptor OA according to embodiments of theinvention.

FIGS. 2 a and 2 b illustrate a fabrication process of an optical adaptoraccording to embodiments.

FIG. 3 depicts another embodiment of the optical adapter OA.

FIG. 4 depicts an arrangement according to embodiments.

FIGS. 5 a, 5 b and 5 c represent detailed views of a photonic chipaccording to embodiments.

FIGS. 6 a, 6 b and 6 c represent detailed views of a photonic chipaccording to another embodiment of the invention.

Most features visible in the appended drawings are meant to be to scale.

DETAILED DESCRIPTION

According to a first aspect, the invention can be embodied as an opticaladaptor monolithically integrating optical elements and firstmicro-mechanical features, the latter defining at least a firsthorizontal reference surface and a first vertical reference surface,wherein, the first horizontal reference surface is perpendicular to anoptical plane, the latter being perpendicular to the optical axis of theoptical elements; and the first vertical reference surface isperpendicular to the first horizontal reference surface and parallel tothe optical axis. In other words, the three reference surfaces definedabove, i.e., the first horizontal reference surface, the first verticalreference surface and the optical plane, are perpendicular two by two.Accordingly, the optical elements can be aligned with waveguides of aphotonic chip upon contacting the first horizontal reference surface andthe first vertical reference surface with corresponding secondhorizontal surface and second vertical surface of the photonic chip.

In embodiments of the invention, the optical adapter may comprise one ormore of the following features: the optical elements comprise a lensarray, the optical plane being perpendicular to optical axes of the lensarray; the optical elements comprise at least two groups of opticalelements of distinct types; at least one group of optical elementcomprises a mirror.

According to another aspect, the invention can be embodied as anarrangement for coupling optical signals comprising an optical adaptoras previously defined and a photonic chip, the photonic chip comprising:waveguides having waveguide terminations at a trench of the photonicchip; and second micro-mechanical features defining at least a secondhorizontal reference surface; and a second vertical reference surface,wherein the second horizontal reference surface is perpendicular to asecond optical plane, the latter being perpendicular to a second opticalaxis of the waveguides, the second vertical reference surface isperpendicular to the second horizontal reference surface and parallel tothe second optical axis, the first horizontal reference surface is incontact with the second horizontal reference surface and the firstvertical reference surface is in contact with the second verticalreference surface.

In embodiments, the arrangement may comprise one or more of thefollowing features: the photonic chip is according to silicon oninsulator technology; a first distance between the first horizontalreference surface and a reference point associated with the opticalelement is equal to a second distance between the second horizontalreference surface and a corresponding reference point associated withthe waveguide, and a third distance between the first vertical referencesurface and the reference point is equal to a fourth distance betweenthe second vertical reference surface and the corresponding referencepoint. Both the first distance and the second distance are equal tozero.

According to another aspect, the invention can be embodied as a methodfor aligning optical elements of an optical adaptor as previouslydefined with waveguides of a photonic chip, the method including puttingin contact the first horizontal reference surface and the first verticalreference surface with corresponding second horizontal surface andsecond vertical surface of the photonic chip.

In embodiments, the method may comprise one or more of the followingfeatures: putting in contact the first horizontal reference surface withthe second horizontal reference surface; and putting in contact thefirst vertical reference surface with the second vertical referencesurface. The photonic chip is fabricated according to silicon oninsulator technology, whereby a silicon on insulator layer stack isobtained. The second micro-mechanical features are fabricated by etchingprocesses selectively ending at interfaces between surfaces of thesilicon on insulator layer stack. Putting in contact comprises settingthe first distance equal to the second distance, preferably equal tozero, and the third distance equal to the fourth distance.

According to yet another aspect, the invention can be embodied as amethod of fabrication of an optical adapter as described above. Inembodiments, the optical element and the first micro-mechanical featuresare obtained through wafer-level processing.

As an introduction of the following description, it is first pointed ata general aspect of the invention, which concerns a method for aligningoptical elements of an optical adaptor with waveguides of a photonicchip.

The optical adaptor monolithically integrates optical elements and firstmicro-mechanical features defining at least a first horizontal referencesurface and a first vertical reference surface. The first horizontalreference surface is perpendicular to an optical plane, which is itselfperpendicular to (and defined by) the optical axis of the opticalelements, and the first vertical reference surface is perpendicular tothe first horizontal reference surface (and parallel to the opticalaxis). The second horizontal reference surface is perpendicular to asecond optical plane, which is itself perpendicular to (and defined by)a second optical axis, i.e., the optical axis of the waveguides. Thesecond vertical reference surface is perpendicular to the secondhorizontal reference surface and parallel to this second optical axis.

The method includes putting in contact the first horizontal referencesurface with the second horizontal reference surface and putting incontact the first vertical reference surface with the second verticalreference surface. The photonic chip may have waveguide terminations ata trench thereof and second micro-mechanical features defining at leasta second horizontal reference surface and a second vertical referencesurface.

FIG. 1 illustrates an optical adaptor OA according to embodiments. Thisoptical adaptor OA monolithically integrates optical elements OE, e.g.,lenses, and micro-mechanical features which define at least tworeference surfaces VP, HP perpendicular to each other. A referencesurface HP is horizontal, i.e., perpendicular to the optical plane OPdefined by the optical axis Ox of the optical elements OE. This opticalplane is by definition perpendicular to this optical axis. In FIG. 1,the optical plane is actually represented by a plane segment OP that isotherwise parallel to a front pane FP of the adapter OA, which frontpane FP supports the optical elements OE. Another reference surface VPis vertical, i.e., perpendicular to the horizontal surface HP. It isalso parallel to the optical axis (i.e., the optical axis does notintersect the plane defined by the vertical reference surface VP).

The optical elements OE may for instance comprise a lens array, as inthe embodiment of FIG. 1, but other types of optical elements may alsobe used, like deflectors, fibers, etc. More generally, the opticalelements shall preferably form a group of individual optical elements.In embodiments, the optical elements OE comprise at least two groups ofoptical elements. These groups can be of the same type or of differenttypes, for instance the optical elements can comprise a group comprisinglenses, and another group comprising a mirror.

As discussed above, the micro-mechanical features (and the referenceplanes they define) are specifically positioned with respect to theoptical elements OE. The optical elements can for instance be associatedto a reference point RP, e.g., the center of the optical elements OE.The micro-mechanical features (and the reference surfaces VP, HP theydefine) can thus be regarded as being specifically positioned withrespect to this reference point.

More in detail, the relative position of the micro-mechanical featureswith regard to the optical elements can be defined by a set ofgeometrical parameters. In the embodiment of FIG. 1, these parametersinclude: a distance d_(VP), which corresponds to the distance betweenthe vertical surface VP and the reference point RP; and a distancebetween the horizontal surface HP and the reference point RP.

In the example of FIG. 1, the horizontal surface HP is aligned with thereference point RP (i.e., the reference point is within the planedefined by the horizontal surface). The distance is then equal to zero.In other embodiments, the distance may be different to zero and thehorizontal surface can be at a small offset of the parallel planecomprising the reference point RP. Other shapes of the optical adapterOA may lead to determine other geometrical parameters defining therelative position of the micro-mechanical features with regard to theoptical elements OE. This allows the reference surfaces being adapted sothat when put in contact with corresponding reference planes of aphotonic chip, the optical elements OE are aligned with the waveguidesWG comprised in this photonic chip. This will be made clearer later inthe description that follows.

FIGS. 2 a and 2 b illustrate the fabrication process of an opticaladaptor according to this embodiment of the invention. FIG. 2 a shows abird's eye view and FIG. 2 b shows a front view of the optical adapterOA after fabrication. In this embodiment, both the lens array OE and themicro-mechanical features can be obtained through wafer-levelprocessing, preferably through etching process on a wafer surface.

The figures show an array of 3×8 optical adapters OA, but much moreoptical elements can be fabricated on a same process, as suggested bythe dotted lines in FIG. 2 a and FIG. 2 b. With such a technique, themicro-mechanical features and the optical elements OE can belithographically positioned with regard to each other. This enables toreach a relative positioning accuracy less than 500 nm in the lateraland vertical directions. Once fabricated, the optical adapters OA can beseparated into individual units.

FIG. 3 depicts another embodiment of the optical adapter OA according tothe invention. It also comprises optical elements OE. These opticalelements comprise two groups. A first group is made of a plurality ofoptical lens LE, and another group is constituted by a deflectingelement (or mirror) DE. A reference point RP is determined, for instanceas being the center of the optical lens LE. With regard to thisreference point RP, micro-mechanical features can be defined. Thesemicro-mechanical features can for instance define a vertical surface VPand a horizontal surface (not depicted in FIG. 3) as being the hiddeninferior face of the optical adaptor OA. In the present case the hiddeninferior face corresponds to the “backside” of the optical lens array.

The optical adapted OA of this embodiment can be fabricated in a similarway as in previous embodiments. Similar accuracy can be reached for therelative positioning of the micro-mechanical features with regard to theoptical elements OE. Such an optical adapter OA can perform anout-of-plane optical coupling, as to be described later in more detail.

FIG. 4 depicts an arrangement according to embodiments. This arrangementcan comprise a photonic chip PC and an optical adapter OA. In accordancewith an exemplary embodiment of the invention, the photonic chip may beaccording to silicon photonic chip in silicon-on-insulator (SOI)technology, so as to obtain a silicon on insulator layer stack. Siliconon insulator technology refers to the use of a layered siliconinsulator/silicon substrate in place of conventional silicon substrates.In SOI-based devices, the silicon junction is above an electricalinsulator, e.g., silicon dioxide or sapphire, or silicon-on-sapphire(SOS).

SOI wafers are widely used for silicon photonic devices. The crystallinesilicon layer can be sandwiched between the buried insulator and topcladding (of air, silicon oxide and any other low refractive indexmaterial). This enables propagation of electromagnetic waves in thewaveguides on the basis of total internal reflection.

The photonic chip PC comprises waveguides WG. These waveguides haveterminations at a trench T of the photonic chip. In embodiments, atrench is formed at the edge to the photonic chip PC. An optical adapterOA can be inserted into the trench. The trench T can be etched withinthe photonic chip PC and the waveguides terminations can be formed atthis trench T. In case the photonic chip is a silicon photonic chip insilicon-on-insulator (SOI) technology, the waveguides WG may be includedwithin the SOI layer stack. In the example of FIG. 4, four trenches havebeen represented, each on a distinct side of the chip.

FIGS. 5 a and 5 b represent detailed views of the circular area labeled“A” in FIG. 4. In FIG. 5 a, an optical adapter AO has been inserted intothe trench T. The optical adapter OA can for instance be the same asdescribed earlier in reference to FIG. 1. The trench T has a shapeadapted to the optical adapter OA. It should allow the optical adapterOA to be inserted. For instance, the depth of the trench should beenough for the optical adapter OA to be inserted enough so that theoptical elements OE can be positioned in front of the waveguides WG. Asit will be seen later, it can be deeper, since the alignment of theoptical adapter OA with regard to the photonic chip will be provided bymicro-mechanical features of the photonic chip PC (and not by the depthof the trench T).

The waveguides are more apparent in FIG. 5 b for instance, where theoptical adapter OA has been removed. The terminations are formed at theend of the photonic chip, in the internal face of the trench T.

FIG. 5 c shows a more detailed view of a part of the trench T and ofmicro-mechanical features comprised with the photonic chip PC. Themicro-mechanical features define reference surfaces CHP, CVP. In thisembodiment of the invention, a horizontal surface CHP and a verticalsurface CVP are defined. The horizontal reference surface CHP isperpendicular to an optical plane defined by the optical axis of thewaveguides WG. This optical plane is by definition perpendicular to thisoptical axis. The vertical reference surface CVP is perpendicular tothis horizontal reference surface and parallel to the optical axis ofthe waveguides WG.

The horizontal surface CHP can for instance be obtained by an etchingprocess of the SOI layer stack, e.g., the etching process selectivelystops at the interface between subsequent layers of the SOI layer stack.The vertical surface CVP can notably be obtained using vertical etchingprocess into the SOI layer stack, preferably into the back-end-of theline (BEOL) structure on top of the SOI layer stack. Themicro-mechanical features are positioned with regard to waveguideterminations and, as a consequence, the reference surfaces CHP, CVP theydefine are positioned accordingly. The waveguides WG can be associatedwith a reference point CRP. This reference point can be the terminationof the central waveguide among all the waveguides WG. The positioning ofthe micro-mechanical features (and the reference surfaces they define)can be made with respect to this reference point.

The relative position of the micro-mechanical features with regard tothe optical elements OE is captured by a set of geometrical parameters.In the embodiment of FIG. 5 c, these parameters comprise a distanced_(CVP) between the vertical reference surface CVP and the referencepoint CRP, and a distance between the horizontal reference surface CHPand the reference point CRP.

The distance d_(CVP) is depicted in FIG. 5 b by a double-headed arrow,between the dotted lines: one dotted line denotes a prolongation of thecentral waveguide, the other dotted line being the prolongation of thehorizontal and vertical reference surfaces CHP, CVP respectively. Bothdistances are determined such that the optical elements OE are alignedwith the waveguides WG when the optical adapter OA is positioned on thephotonic chip PC. The accuracy that can be achieved in the positioningof the horizontal surface CHP, which is determined by the accuracy ofthe vertical position of the waveguides into the SOI layer-stack, can beless, i.e., better than 10 nm. The accuracy of the positioning of thevertical surface CVP can be less than 500 nm, e.g., in embodiments wherethe horizontal position of the BEOL wall (with respect to the waveguideposition) is lithographically defined.

Referring back to FIG. 5 a, it will be seen how the micro-mechanicalfeatures of the optical adaptor OA and of the photonic chip PC definesreference surfaces HP, VP, CHP, CVP enabling lateral and verticalalignment of the optical adaptor and of the photonic chip so that thewaveguides WG can be aligned with the optical elements OE. This enablesoptical in-plane coupling of the waveguides WG with the optical elementsOE.

More precisely, in this embodiment, the horizontal surface HP of theoptical adapter OA is put in contact with the horizontal surface CHP ofthe photonic chip PC. This aligns the optical elements OE of the opticaladapter OA with the waveguides WG according to the elevation axis. Thevertical surface VP of the optical adapter OA is put in contact with thevertical surface CVP of the photonic chip PC. This aligns the opticalelements OE of the optical adapter OA with the waveguides WG accordingto the horizontal axis, perpendicular to the optical axis.

In this embodiment, putting in contact the reference surfaces maycomprise setting the previously-defined distances so that: a firstdistance, which is the distance between the horizontal reference surfaceHP and the reference point RP associated with the optical elements, isequal to a second distance, which is the distance between the secondhorizontal reference surface CHP and the corresponding reference pointCRP associated with the waveguide WG, and a third distance, i.e., thedistance d_(VP) between the vertical reference surface VP and thereference point RP is equal to the distance d_(CVP) between the secondvertical reference surface CVP and the corresponding reference pointCRP.

Note that the first and second distances (e.g., equal) are determined bythe distance between the center of the waveguide and the upper surfaceof the buried oxide (BOX) layer in the SOI layer stack. This distancedepends on the height itself of the silicon waveguide. Typical distancesare about 220 nm/2=110 nm. More generally, the distances can be a fewhundreds of nanometer (but preferably less than 1000 nm).

The third (d_(vp)) and fourth (d_(cvp)) distances (e.g., equal) dependon the number of waveguides and the pitch. For instance, FIG. 5represents 10 waveguides with a pitch of 125 μm. Thus, the distancesapproximately correspond to 9×125 μm/2+60 μm (radius of a lens)+100 μm(clearance lens-to-feature)+100 μm of the HP extension=822.5 μm.Roughly, for 10 waveguides with 125 μm pitch, the distances areapproximately equal to 1 mm. Yet, one understands that even moderatevariations in the previous parameters may result in distances between0.5 and 2 mm. Of course, depending on the number of waveguides and thepitch, still other ranges of distances could be obtained.

Next, the area of the horizontal surfaces HP (and correspondingly theCHP) is preferably on the order of 100 μm×200 μm. The area of thevertical surfaces VP (and correspondingly the CVP) is preferably on theorder of 15 μm×200 μm. More generally, each of the above areas can bemeasured as a×b, where each of a and b is typically between 15 and 500μm.

FIGS. 6 a, 6 b, 6 c illustrate another embodiment, where the opticaladapter OA can for instance be the same as described earlier inreference to FIG. 3. They are detail views of a photonic chip PC asrepresented in FIG. 1.

FIG. 6 a shows a trench T which has a different shape, compared to FIG.5, adapted to the shape and function of the optical adapter OA. In FIG.6 b, the optical adapter OA is put in place inside the trench T.

FIG. 6 c is a magnified view, where the horizontal surface CHP andvertical surfaces CVP are visible.

As described earlier, the horizontal surface of the optical adapter OAcan be put in contact with the horizontal surface CHP and the verticalsurface of the optical adapter OA put in contact with the verticalsurface CVP of the photonic chip PC. In this way, the optical elementsOE can be precisely aligned with the waveguide WG, with similar accuracyas in previous embodiments. This enables optical out-of-plane couplingof the waveguides WG with the optical elements OE.

While the present invention has been described with reference to alimited number of embodiments, variants and the accompanying drawings,it will be understood by those skilled in the art that various changesmay be made and equivalents may be substituted without departing fromthe scope of the present invention. In particular, a feature(device-like or method-like) recited in a given embodiment, variant orshown in a drawing may be combined with or replace another feature inanother embodiment, variant or drawing, without departing from the scopeof the present invention. Various combinations of the features describedin respect of distinct embodiments or variants may accordingly becontemplated, that remain within the scope of the appended claims. Inaddition, many minor modifications may be made to adapt a particularsituation or material to the teachings of the present invention withoutdeparting from its scope. Therefore, it is intended that the presentinvention not be limited to the particular embodiments disclosed, butthat the present invention will include all embodiments falling withinthe scope of the appended claims. In addition, many other variants thanexplicitly touched above can be contemplated. For example, otherarrangements of waveguide facets can be considered such as suspendedwaveguide-ends by substrate removal. Other fabrication processes thansilicon-on-insulator technology can be contemplated, such as III-Vphotonics co-integrated on SOI layer stack.

The invention claimed is:
 1. A system for coupling optical signals,comprising: an optical adaptor having monolithically integrated opticalelements and first micro-mechanical features, the first micro-mechanicalfeatures defining at least a first horizontal reference surface and afirst vertical reference surface, wherein the first horizontal referencesurface is perpendicular to an optical plane, the optical plane beingperpendicular the optical axis of the optical elements, and the firstvertical reference surface is perpendicular to the first horizontalreference surface and parallel to the optical axis; and a photonic chip,comprising waveguides having waveguide terminations at a trench of thephotonic chip, and second micro-mechanical features defining at least asecond horizontal reference surface and a second vertical referencesurface; wherein the second horizontal reference surface isperpendicular to a second optical plane, the second optical plane beingperpendicular to a second optical axis of the waveguides, the secondvertical reference surface is perpendicular to the second horizontalreference surface and parallel to the second optical axis, the firsthorizontal reference surface is in contact with the second horizontalreference surface and the first vertical reference surface is in contactwith the second vertical reference surface.
 2. The system of claim 1,wherein the photonic chip comprises silicon-on insulator (SOI)technology.
 3. The system of claim 1, wherein a first distance betweenthe first horizontal reference surface and a reference point associatedwith the optical element is equal to a second distance between thesecond horizontal reference surface and a corresponding reference pointassociated with the waveguide, and wherein, a third distance between thefirst vertical reference surface and the reference point is equal to afourth distance between the second vertical reference surface and thecorresponding reference point.
 4. The system of claim 3, wherein each ofthe first distance and the second distance is equal to zero.